Triple Junction Stability

SO461 Lab, Fall 2008

Plate Motions and Triple Junction Stability

This analysis is also covered on pages 95-101 in the text. You should read that before the lab, and bring the text to the lab.

Bring a ruler and protractor to the lab. You may also want to bring graph paper.

Insure that you understand the difference between relative and absolute plate motions. For this lab we will deal with the relative motions between pairs of plates.

You may do this lab in groups of no more than 3. It will be due Tuesday 10/28, and will not be accepted late without explicit prior permission..

Open MICRODEM (update your computer to insure that you have the current version).
Open the help file, and read the section on Plate Vector Motions--it is on the table of contents, under "Tutorials, Plate Tectonics". It contains analyses of the Indian Ocean triple junction and the Gulf of California triple junctions. Note that it describes using geometric and analytic solutions . You should use the geometric option.
You will analyze the following triple junctions. You should do this with the NUVEL, No Net Rotation plate model since it has more plates included.
- The Nazca--Cocos--Pacific triple junction
- The triple junction at S45, W76

Graded Requirements:

1. For each triple junction you will turn in:

Velocity stability diagram. It should be approximately correct, but you can estimate the velocities and angles.
Nature of the triple junction (ridge-ridge-ridge, etc.)
Is the triple junction stable, and if so, will it be migrating. If it is not stable, how large a change would be required to make it stable.
Include a map with the ages of the seafloor, and an elevation map from Smith and Sandwell to explain the relationships.

2. Discuss the relationship between deep earthquakes and plate convergence. Include a map showing where you made the computations, and the plates involved. You should filter the earthquake focal mechanisms to help you pick the locations.

Find two trenches with earthquakes extending down to 500 km. Compute the plate convergence at those boundaries, and the direction of the convergence compared to the plate boundary.
Find two trenches with no earthquakes extending below 100 km. Compute the plate convergence at those boundaries, and the direction of the convergence compared to the plate boundary.

Map of the plates

Plate motion calculators, available if you do not want to use MICRODEM.

Previous Triple junctions

The Philippine--Pacific--Eurasian triple junction SE of Japan. Some scientists think the Eurasian--North American plate boundary is west of its location on the map in MICRODEM (for instance Map of the plates), and that this triple junction is actually Philippine--Pacific--North America. Repeat the calculations for this configuration, and does it make much difference (i.e. what are the relative plate velocities between North America and Eurasia in this region?) Repeat the calculations for the APKIMA plate model.